In order to understand a developmental switch with which phage lambda decides its fate, lytic growth vs. lysogeny, we wish to study in vitro the regulatory proteins involved. The three-dimensional structure of Cro repressor is worked out at 2.8 A resolution. This has to be refined further to locate amino acid side chains. Genetic experiments and chemical modifications of Cro repressor will be continued to know the functional groups involved in specific and non-specific DNA binding. In particular, chemical modification of tyrosine, histidine, arginine and lysine will be used to map Cro repressor for DNA binding sites. A lambda operator sequence will be chemically synthesized and used for co-crystallization with Cro repressor which will be used for X-ray crystallographic analysis. All this information will be used to make a model of repressor-operator interaction. The positive regulator cII protein will be purified. This will be used to isolate cII protein and host factors using an affinity column. Pure cII protein will be used for chemical modification as above to learn the functional groups for DNA interaction.